During friction stir welding, two pieces of metal can be mechanically intermixed at the place of the join. During intermixing, the materials can be transformed into a softened state that allows the metal to be fused using mechanical pressure. A friction stir weld is made with a rotating tool that is plunged into a material along a joint line, and then translated along the joint. The tool, therefore, is preferably wear-resistant and exhibits high strength, toughness, oxidation resistance, and low thermal conductivity. Widespread commercial applications of friction stir welding will require the development of cost-effective and durable tools.